The present invention relates to a sputtering target that can decrease or prevent cracking of a target occurring during bonding of the target to a backing plate or during the use of the target.
In recent years, sputtering is used for forming thin films of semiconductor devices and various electronic apparatuses, and sputtering of higher speed is frequently used for improving productivities.
As is well known, sputtering described above is a method for forming thin films by radiating charged particles onto a target, driving out particles from the target by impact force of the charged particles, and depositing a thin film mainly comprising a substance composed of target material on a substrate such as a wafer facing the target.
Since the target receives a large quantity of impact of charged particles during sputtering, the temperature of the target gradually increases and accumulates. Therefore, the target must be cooled. In many cases, the heat of the target is absorbed by bonding a material of high thermal conductivity (backing plate) such as pure copper and copper alloys on the back of the target by means of brazing, diffusion bonding, pressing, or bonding that utilizes an anchoring effect, and cooling the backing plate through an external cooling means.
As described above, when the target is bonded to the backing plate, the target must be heated to a temperature suitable for various bonding methods, and after bonding, it is cooled. An effect of heat from heating and cooling processes in bonding causes a problem of warping or cracking due to a difference in thermal expansion between the target and the backing plate.
Since a cracked target is defective, it must be replaced with a target without cracking. At this time, the target formed by combining a plurality of pieces has possibility to cause the piece without cracking to be cracked, lowering productivity significantly.
Even if the a target is bonded to a backing plate satisfactorily, since the assembly is heated also during sputtering, the target may be cracked by thermal stress.
In such a case, a large number of particles are produced when the target is cracked, and serious problems may be caused in that the film becomes defective, sputtering cannot be continued due to the damage of the target, or the sputtering apparatus is seriously damaged.
When the type of the target material is considered, since a ceramic target is brittle, has a small coefficient of thermal expansion, and has a large difference in the coefficient of thermal expansion from that of the material of the backing plate such as pure copper or copper alloys, warping or cracking occurs easier than metal targets such as Al and Ti.
In order to solve such problems, the use of a low-melting-point solder has been proposed for bonding so that the target and the backing plate are not exposed to high temperature. In this case, however, the bonding force is weak, and when the target is heated to a high temperature during sputtering, there is the problem that the solder for bonding is melted, and this problem has not been solved.
Also, in order to relieve the thermal expansion of the target and the backing plate, the formation of a plurality of bonded layers with stepwise different thermal expansion has been proposed (Japanese Patent Laid-Open No. 61-250167). However, such means is not practical because it complicates an operation process and increases manufacturing costs.
Furthermore, a method for correcting a warped target by applying mechanical force in the reverse direction in a process for bonding the target and the backing plate, or a method for applying reverse warping to the target before bonding estimating the amount of warping has been proposed (Japanese Patent No. 2573653).
However, since mechanical stress more than material strength is applied to the brittle material such as a ceramic, in particular, in a process of mechanical correction or deformation, cracking may occur, and these cannot be said to be effective means.
The inventors of the present invention found a unique phenomenon in the manufacturing process that there is a direction in which cracking due to a warp of the target occurs easily in a target, especially in a ceramic target. The inventors made effort to provide a sputtering target that can effectively decrease or prevent cracking of the target during bonding the target to the backing plate or during sputtering process.
The present invention pays attention to the ordinary process for grinding or polishing the target, and prevents or decrease cracking of the target by improving the operations. According to the present invention there are provided: (1) a sputtering target characterized in that a part of a direction or a tangential direction of grinding a top surface or both surfaces of the target is parallel to, or within an angle range of xc2x145 degrees to a warping direction of the target after bonding the target to a backing plate; (2) a sputtering target characterized in that the target is rectangular, and a part of a direction or a tangential direction of grinding a top surface or both surfaces of the target is within a range of angles formed by a warping direction after bonding the target to a backing plate and a diagonal of the target; (3) a sputtering target characterized in that the target is rectangular, and a part of a direction or a tangential direction of grinding a top surface or a both surfaces of the target is within a range of angles formed by a lengthwise direction and a diagonal of the target; (4) the sputtering target according to any of (1) to (3), characterized in that a final surface after grinding is a surface improved by fine grinding or polishing, or acid washing or heat treatment; (5) the sputtering target according to any of (1) to (4), characterized in that the surface roughness Ra is 5 xcexcm or less; (6) the sputtering target according to any of (1) to (5), characterized in that the surface roughness Ra is 2 xcexcm or less; and (7) the sputtering target according to any of (1) to (6), characterized in that the target is made of a ceramic.